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Different IP's with same MAC address


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#1 rkwon

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Posted 08 August 2012 - 01:43 PM

Hopefully someone can help me with this problem...

I volunteer my time at a church to do light management of their intranet network. I am not a network expert so there is a limit to what I can do.

The other day, the church's intranet went down. The main error message I was seeing was related to "IP conflict".
I used a free network scan tool to scan the intranet and came up with a weird result.

Basically, what I found was that different computers (with different IP's - both static and dynamic IP's) had same MAC address. The MAC address in question is all same and is belong to a Amped Wireless equipment (F8:7B:8C).

As the problem was getting worse, additional computer's MAC address was being corrupted with the same MAC address (about 10 computers per minute).

What would cause something like this?

The network in question has a Netgear FVS336G v2 as the firewall/VPN router and a Netgear switch (not sure which model). It is connected to the internet by Comcast business line. It also has 2 wireless AP's (all Cisco - SMB model) connected to it as well.

Here is the picture of the scan:
http://imgur.com/PxcaT

Edited by rkwon, 08 August 2012 - 01:52 PM.


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#2 Sneakycyber

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Posted 08 August 2012 - 05:12 PM

Grab any pc on the network Log on as a domain admin or System Administrator and start by typing this into Windows explorer
\\10.100.10.30\c$
Navigate to Documents and settings and find the culprit PC. If you can't determine the PC then the safest and the fastest way to solve the issue is to shut down the network and power it back up one piece at a time. Starting with the Modems, then router, lastly File server or a Trusted PC. Do a network scan. Power up systems one at a time until the problem resurfaces. The Amped Wireless is a USB adapter. Likely it has been damaged or the PC its connected to keeps renewing its Ip address and the router keeps assigning it a new one.

Edit 3. Depending on the size of the network shut down may not be an option. If that is the case disable the DHCP server and start looking for the problem PC or Networked device.

Edit 4. Block the Mac address in the Router wait for someone to complain about loss of internet. (I swear that's the last edit)

Edited by Sneakycyber, 08 August 2012 - 05:26 PM.

Chad Mockensturm 

Systems and Network Engineer

Certified CompTia Network +, A +





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